Sentinels are robotic entities that guard most worlds. Harvesting resources in view of the Sentinels will cause them to turn hostile.
Harvesting some resources, such as Gravitino Balls, will immediately alert Sentinels forces.
Sentinels may approach and scan you or any resources you recently harvested. Avoid harvesting or attacking non-hostile creatures while being watched.
Some worlds host extremely hostile Sentinels. These drones will attack on sight. Pay attention to the planet information in the Analysis Visor (Key F) to learn Sentinels status.
The Security Level in the top right of the HUD shows the current Sentinels status. This will increase as you fight Sentinels.
Eliminate Sentinels Drones quickly when to avoid more powerful Sentinels being summoned.
Destroying Sentinel Drones is a good source of exotic materials.